Supporting Information to Microgel Size Modulation by Electrochemical Switching

Olga Mergel, a Patrick Wünnemann, a Ulrich Simon, b Alexander Böker, c Felix A. Plamper a* a Institute of Physical Chemistry II, RWTH Aachen University, Landoltweg 2, 52056 Aachen, Germany b Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany c Fraunhofer-Institut für Angewandte Polymerforschung (IAP), Lehrstuhl für Polymermaterialien und Polymertechnologie, Universität Potsdam, Geiselbergstraße 69, 14476 Potsdam, Germany

*Corresponding Author: Felix A. Plamper ([email protected]; +49 241 8094750)

1. Materials and Methods

Materials. The microgel particles were synthesized by precipitation copolymerization of N-isopropylacrylamide (NiPAM) and N-[3-(dimethylamino)propyl]methacrylamide (DMAPMA) and subsequent quaternization of the DMAPMA comonomer amine function with methyl iodide. A detailed microgel synthesis and characterization is 1 described elsewhere. Potassium chloride (KCl) and potassium hexacyanoferrate(III) (K 3[Fe(CN) 6]) were obtained . from Merck. Potassium hexacyanoferrate(II) trihydrate (K 4[Fe(CN) 6] 3H2O) was purchased from AnalaR NORMAPUR. All chemicals were used as received without any further purification. Deionized water (18.2 MΩ) from Millipore Milli-Q-purification system was distilled twice and used in all experiments.

General Electrochemical Techniques. All electrochemical measurements were conducted on the CH Instruments Electrochemical Workstation Potentiostat CHI760D (Austin, Texas, USA). The experiments were performed in a conventional three-electrode setup in a water jacketed cell connected to a thermostat (thermo scientific Haake A28) at 20 °C or at 36 -37 °C in case of electrolysis experiments and at room temperature (23 °C) in case of hydrodynamic voltammetry experiments (for concentration determination of supernatant) and electrochemical impedance spectroscopy. For such hydrodynamic voltammetry experiments, the potentiostat was connected with a rotating ring disk rotator (RRDE-3A from ALS Japan). Two kinds of working electrodes have been used. For hydrodynamic voltammetry experiments, a platinum (rotating) disk electrode, 4 mm disk diameter, was conducted as working electrode. For bulk electrolysis experiments, a platinum gauze electrode (35 mm x 20 mm) was used as working electrode. Furthermore, a spirally platinum electrode, 23 cm (legth), was used for hydrodynamic voltammetry experiments and a platinum wire (50 mm) for electrolysis as counter electrode, which was immersed in 0.025 M, or 0.05 M KCl solution separated by a diaphragm from the remaining compartment (2 half-cell setup). For electrochemical impedance spectroscopy experiments, the platinum gauze electrode served as counter electrode, while the rotating disk electrode (without rotation) was connected as working electrode. An Ag/AgCl electrode stored in 1 M KCl served as reference electrode in all three cases. All potentials in the text and figures are referenced to the Ag/AgCl couple. Electrolysis experiments of an initial solution of 1 mM K 4[Fe(CN) 6] in a supporting electrolyte solution of 0.025 M KCl were performed in presence of the microgel P(NIPAM-co - MAPTAC) (6.3 g / L; 0.63 wt%) with an initial charge ratio of icr = 1, or the double concentrated solution of 2 mM K4[Fe(CN) 6] in 0.05 M KCl in presence of P(NIPAM-co-MAPTAC) microgel (12.5 g / L; 1.25 wt%, icr = 1). The solutions were purged with Ar for 10 min to remove dissolved oxygen.

Bulk electrolysis experiments were performed at 20 °C or ~ 37 °C by application of an oxidation potential of 0.6 V to an initial solution containing 0.63 wt% of P(NIPAM-co -MAPTAC) microgel and 1 mM K 4[Fe(CN) 6] in 0.025 M KCl with an initial charge ratio ( icr ) of 1. For stepwise oxidation a charge of -12 to -20 mC for diluted microgel dispersion and -125 to -177 mC for concentrated dispersions was transferred and the microgel size was determined by 3D DLS experiments. This stepwise oxidation was repeated 9 - 10 times. For reduction, a potential of 0 V was applied to the K 3[Fe(CN) 6] containing microgel dispersion. Furthermore, full oxidation and reduction were performed by transfer of ±0.9 C in diluted dispersions (1 mM K 4[Fe(CN) 6]) and ±1.8 C in the concentrated case (2 mM K 4[Fe(CN) 6]). 1

Rotating Disk Electrode (RDE). In order to obtain information about the nominal charge ratio ( ncr ) and thus the amount of the effectively incorporated multivalent counterions, centrifugation experiments (40.000 rpm; 30 °C) of an initial solution of 1 mM K 4[Fe(CN) 6] in 0.025 M KCl in presence of 6.3 g/L P(NIPAM-co -MAPTAC) microgel were performed after oxidation and reduction. Hydrodynamic voltammograms of the supernatant were recorded by sweeping the potential in the range of (-0.1) V - 0.5 V vs. Ag/AgCl at a scan rate of 5 mV s -1. The rotation rate was remained constant at 100 rpm at a temperature of 23 °C. Further, hydrodynamic voltammetry experiments were performed at different electrolysis states: at an intermediate state of 1 mM K 4[Fe(CN) 6], 1 mM K 3[Fe(CN) 6] and at 3- 4- the extreme states of 2 mM [Fe(CN) 6] or 2 mM [Fe(CN) 6] at 37 °C in 0.05 M KCl (100 rpm in presence and absence of microgel; 12.5 g/L P(NIPAM-co -MAPTAC)). Before performing each hydrodynamic voltammetry measurement, the working electrode was polished first with 1 µm diamond and subsequently with 0.05 µm alumina polish, rinsed with water and dried with a stream of argon.

Electrochemical Impedance Spectroscopy (EIS) experiments were performed at different electrolysis states. An initial solution of 2 mM K4[Fe(CN) 6] in 0.05 M KCl in presence and absence of the microgel (12.5 g/L P(NIPAM-co - MAPTAC) was oxidized at 37 °C, while impedance spectroscopy experiments were performed in steps of -300 mC 4- transferred charge. In order to measure the extreme stages of switching, 2 mM [Fe(CN) 6] in 0.05 M KCl, or 2 mM 3- [Fe(CN) 6] in 0.05 M KCl (in presence and absence of microgel) were prepared freshly and were not electrolyzed before the impedance spectroscopy experiment. The (dc) potential was held at the open circuit potential measured at each electrolysis stage, while a small oscillating voltage of 5 mV amplitude was applied (leading to an alternating current – ac – readout). The measuring frequency f used for EIS measurements ranged from 1 Hz to 100 kHz. As fit model, the modified Randles circuit was used, in accordance to our previous publication. 1 Impedance data analysis was performed according to proper transfer function derivation and identification procedures, which involved complex nonlinear last-squares (CNLS) fitting based on the Marquardt-Levenberg algorithm using the CH Instruments Beta software.

Electrophoretic mobility . Measurements of the electrophoretic mobility were performed on a NanoZS Zetasizer (Malvern). Zeta-Potential was derived by use of the Smoluchowski limit. Measurements were performed in disposable capillary cells (Malvern, DTS1061C). Electrophoretic mobility was measured at an angle of 17° at a wavelength of the laser beam was 633 nm. Measurements were performed at 25 °C.

3D cross correlation dynamic light scattering (3D-DLS) setup was used for the determination of the hydrodynamic radii, as the samples were turbid. All experiments were performed on an LS instruments setup (Fribourg, Switzerland) equipped with a 633 nm HeNe laser (JDS Uniphase, KOHERAS GmbH, 25 mV, Type LGTC 685-35), a goniometer (ALV, CGS-8F), digital Hardware correlator (ALV 7004, ALV GmbH, Langen, Germany), two avalanche photo diodes (Perkin Elmer, Type SPCM-AQR-13-FC), light scattering electronics (ALV, LSE-5004), an external programmable thermostat (Julabo F32) and an index-match-bath filled with toluene. Angle- and temperature-dependent measurements were recorded in pseudo-cross correlation mode varying the scattering angle from 45 to 105 ° at 5 ° intervals including a variation of temperature (in the range of 20 to 60 °C at 2 K intervals for temperature-dependent experiments and measurement time of 200 s). For experiments at a constant temperature of ~ 37 °C, the scattering angle was varied from 45 to 102 ° at 3 ° intervals. For data evaluation, the decay rate (first cumulant) from second order cumulant fit was plotted against the squared length of the scattering vector q2. The data were fitted with a homogeneous linear regression, whereas the diffusion coefficient was extracted from the slope and the hydrodynamic radius Rh calculated by using the Stokes-Einstein equation.

Scanning Force Microscopy (SFM). The swelling behavior of the microgels was observed with a liquid-cell AFM (Veeco Dimension Icon) to mimic bulk solution conditions. A temperature-controlled stage (ICONEC-V2-NOPOT, Bruker AXS) was used to equilibrate the custom-made cell before each measurement for ~ 90 min. The microgels were analyzed with MSCT-A (Bruker AXS) tips (spring constant 0.07 N/m, resonant frequency 22 kHz) at 25 °C and 37 °C via Peak Force QNM. For the liquid-cell experiments, 20 µl of an aqueous dispersion were spincoated at 2000 rpm for 45 s onto a silicon wafer, which was cleaned with toluene (ISO, VWR), dried with a CO 2 jet system and activated via plasma treatment for 30 s (Plasma Activate Flecto 10 USB, 100 W, 0.2 mbar) before deposition. This silicon wafer was immersed in a two-chamber homemade measurement cell. The chambers were separated by a dialysis membrane in order to equilibrate both chambers. One part was filled with double concentration of microgels, whereas the wafer was placed into the other part for SFM investigations. The concentrations of salts (and the average microgel concentration) were basically the same as for the bulk electrolysis experiments. After data extraction from the AFM images via Nanoscope 8.15 (Bruker AXS), the average microgel height profile was 2 calculated from a number of single microgels (typically 6 microgels). The same microgels were used at 25°C and 37°C. For the calculation of the average microgel height profile, only those microgels were considered, whose height deviates not more than +/- 15 nm from the average height at a specific condition.

2. Scanning Force Microscopy (SFM) . The data at 37°C is compiled in Figure S1.

120 4- [Fe(CN) 6] 100 3- [Fe(CN) 6] 80

60

40 Height [nm] Height 20

0

0 200 400 600 800 Cross-sectional axis [nm]

Figure S1. Scanning force microscopy image of P(NIPAM-co -MAPTAC) microgels absorbed onto silicon wafer in liquid 4- 3- state in presence of exclusively 1 mM [Fe(CN) 6] (left; icr = 1), exclusively 1 mM [Fe(CN) 6] (middle; icr = 0.75) in 0.025 M KCl at 37°C and average height profiles across the apex of the absorbed microgel (right).

3. Electrochemical Counterion Switching

During the oxidation process, we aimed to transfer -1.8 C charge during oxidation (see Figure S2, left), indicating an electrochemical conversion of 93 % of the initially 2 mM K 4[Fe(CN) 6]. According to Faraday´s law, Q = n F N 0, (with Q: total amount of transferred charge during the electrolysis, n: number of transferred electrons, F: Faraday 4- constant and N0: the molar amount of electro-converted species) 1.9 mM of the [Fe(CN) 6] were electrochemically 3- transformed into the trivalent counterion [Fe(CN) 6] leading to a new initial charge ratio of icr = 0.77.

2.0 0.0 1.6 4- -0.4 2 mM [Fe(CN) 6] + µG 3- 2 mM [Fe(CN) 6] + µG 4- 2 mM [Fe(CN) 6] 1.2 3- 2 mM [Fe(CN) 6] -0.8 0.8 [C] [C]

Q -1.2 Q 0.4 -1.6 0.0 -2.0 0 5 10 15 20 25 30 35 -10 0 10 20 30 40 50 60 70 t [min] t [min] Figure S2. Transferred charge against time during a nearly full oxidation process (left) and reduction process (right) of an initial solution of 2 mM K4[Fe(CN)6] 0.05 M KCl in presence (c(µG): 12.5 g/L, icr = 1, red) and absence (black) of P(NIPAM-co -MAPTAC) microgel; applied oxidation potential: 0.6 V and reduction potential 0 V.

Interestingly, the oxidation time scale is somewhat shortened in presence of the microgel (especially at the beginning of the electrolysis), while the reduction takes somewhat longer in presence of the microgel (especially at the end of the electrolysis). In analogy of the Butler-Volmer equation, the thermodynamically-preferred incorporation of the product ferricyanide has probably an effect on the kinetics by lowering the energetic level of the transition state (which affects especially the oxidation). This is also reflected in a decrease in the RCT upon microgel addition (main part). At the end of the electrolysis, the slower reduction process in presence of the 3- microgel (Figure S2, right) could still indicate an increased retention of the trivalent [Fe(CN)6] inside the gel, but the accessibility is still assured due to a conversion of > 90%.

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To get an estimate on the influence of concentration, which might lead to glassy/crystalline states obstructing free diffusion of the microgel particles during the light scattering experiment, the electrochemical counterion switching was also performed within a more diluted microgel suspension, whereas the ratio between microgel to counterion to KCl was maintained (Figure S3). The oxidation of the guest molecule leads to a collapse of the microgel in a comparable size range in respect to the more concentrated microgel suspension (indicating a rather unhindered diffusion).

140

135 Oxidation Reduction

130

125 Rh [nm] 120

115 110 0.08 0.12 0.16 0.20 0.24 0.28 0.32 Equilibrium Potential [V vs. Ag/AgCl]

Figure S3. Hydrodynamic radius against the equilibrium potential during stepwise oxidation and reduction of an initial solution of 1 mM K 4[Fe(CN) 6], 0.025 M KCl in presence of 6.3 g/L P(NIPAM-co -MAPTAC) microgel (icr = 1, 3D-DLS at 38 °C); applied oxidation potential: 0.6 V and reduction potential 0 V at 20 °C each.

Figure S2 and S3 are demonstrating the full reversibility of the microgel swelling and collapse induced by the 3- 4- electrochemical switching of the multivalent guest molecule from [Fe(CN) 6] to [Fe(CN) 6] and vice versa.

4. Electrochemical Impedance Spectroscopy

Impedance spectroscopy experiments were performed to gain a deeper mechanistic insight into the charge 3- 4- transport processes within the microgel in presence of exclusively trivalent [Fe(CN) 6] or tetravalent [Fe(CN) 6] and an intermediate state of 1 mM K 4[Fe(CN) 6], 1 mM K 3[Fe(CN) 6 (Figure S4 Nyquist diagram and S5 bode representation). Upon microgel addition, the largest changes in spectra can be seen for ferricyanide-rich solutions.

-12000

-9000 HCF III

] fit

Ω HCF III + µG [ -6000 fit HCF II fit HCF II + µG Imaginary fit

Z -3000 HCF III/ II fit HCF III/II + µG fit 0 0 3000 6000 9000 12000 Z [Ω] Real

Figure S4: Nyquist diagram in presence of 12.5 g/L P(NIPAM-co -MAPTAC) (dark colors) and absence of microgel (light colors) at different electrolysis states (black/grey: 1 mM K 4[Fe(CN) 6], 1 mM K 3[Fe(CN) 6]; green/light green: 2 mM 3- 4- [Fe(CN) 6] ; blue/light blue: 2 mM [Fe(CN) 6] ) at 37 °C; dc: open circuit potential; ac: 5 mV; frequency range: 100 kHz – 1 Hz; full lines are fit data. 4

-80

-70 10000 HCF III -60 HCF III HCF III + µG HCF III + µG HCF II HCF II -50 HCF II + µG

HCF II + µG ]

] HCF III/II

° HCF III/II Ω

[ -40 [ HCF III/II + µG

HCF III/II + µG 1000 ϕ -30 Z

-20

-10 100

0 1 10 100 1000 10000 100000 1 10 100 1000 10000 100000 Frequency [Hz ] Frequency [Hz ]

Figure S5: Bode plots of in presence of 12.5 g/L P(NIPAM-co -MAPTAC) (dark colors) and absence of microgel (light colors) at different electrolysis states (black/grey: 1 mM K 4[Fe(CN) 6], 1 mM K 3[Fe(CN) 6]; green/light green: 2 mM 3- 4- [Fe(CN) 6] ; blue/light blue: 2 mM [Fe(CN) 6] ) at 37 °C; dc: open circuit potential; ac: 5 mV; frequency range: 100 kHz – 1 Hz.

In order to obtain a clearer picture about the possible changes of the electron pathway processes during the whole electrolysis, we performed electrochemical impedance spectroscopy experiment in more refined steps. The transferred charge per step amounts -300 mC.

-1600 -1600 -1.5 C -1.5 C -1.5 C fit -1.5 C fit -1.2 C -1.2 C -1.2 C fit -1200 -1.2 C fit -1200 -0.9 C -0.9 C ] ] -0.9 C fit -0.9 C

-0.6 C Ω -0.6 C Ω [ [

-0.6 C fit -0.6 C fit -800 -0.3 C -800 -0.3 C -0.3 C fit -0.3 C fit Imaginary Imaginary -400 -400 Z Z

0 0 0 400 800 1200 1600 0 400 800 1200 1600 Z [Ω] Z [Ω] Real Real

Figure S6: Nyquist diagram in presence of 12.5 g/L P(NIPAM-co -MAPTAC) (right) and absence of microgel (left) at 4- different electrolysis states (initial solution 2 mM [Fe(CN) 6] in 0.05 M KCl, transferred charge per oxidation step -300 mC at 37 °C; dc: open circuit potential; ac: 5 mV; frequency range: 100 kHz – 1 Hz; full lines are fit data.

. q. q ZCPE = 1/(Q ϖ j )

. 1/2 -1 Zw= (Y 0 ϖ )

Scheme S1: Schematic illustration of the modified Randles circuit with bulk solution resistance RS, the charge transfer resistance RCT , the constant phase element CPE (with parameters Q and q) and Warburg impedance Z w expressed with admittance term Y0 (ϖ assigns here the angular frequency regarding the oscillation of the electrode potential; j assigns the complex number j 2 = -1; Y0 is interconnected to the Warburg coefficient σ by σ = 1/( Y0·2 1/2 )). 5

180 1.0 0.000014

reference 160 0.8 + mg 0.000012

140 ] 0.000010

n 0.6 ]

Ω 0.000008 sec

[ 120 n .

S 0.4 S [

R 100 0.000006 Q 0.2 0.000004 reference 80 + mg

0.000002 0.0 0.0 -0.5 -1.0 -1.5 -2.0 0.0 -0.5 -1.0 -1.5 -2.0

Transferred Charge [C] Transferred Charge [C]

Figure S7: Fitting parameter RS and the constant phase element CPE (with parameters Q and q) against the transferred charge during stepwise oxidation process of 2 mM K 4[Fe(CN) 6 in 0.05 M KCl; in presence of P(NIPAM-co -MAPTAC) 12.5 g/L, icr = 1; red squares) and absence of microgel (black circles).

The data were fitted with a modified Randles circuit shown in Scheme S1 and the fitting parameters are given in Figure S7 and in the main part Figure 7 and 8.

5. Hydrodynamic Voltammetry (Rotating Disk Electrode RDE)

Hydrodynamic voltammetry in combination with centrifugation experiments was used in order to obtain information about the nominal charge ratio ( ncr, molar ratio of the entrapped hexacyanoferrate charges in relation to the microgel charges). Therefore, centrifugation experiments before switching and after oxidation/ reduction were performed. The amount of entrapped counterions was recalculated from hydrodynamic voltammograms of the supernatant and by the help of calibration curves at different concentrations for ferri- and ferrocyanide respectively. The hydrodynamic voltammograms of the supernatant before switching and after reduction are given in Figure S8. As the limiting current iL is proportional to the concentration of the redox probe according to the Levich equation, one can extract the amount of entrapped counterions from the limiting current of the hydrodynamic voltammograms of the supernatant (see Figure S8) at a constant rotation rate. The larger modulus of the iL value (12 µA compared to 9 µA) of the hydrodynamic supernatant voltammogram in presence of the 4- 3- tetravalent [Fe(CN) 6] is a first hint, that more trivalent [Fe(CN) 6] counterions are incorporated into the microgel network. Furthermore, there is no difference between iL before switching and after an oxidation/reduction cycle emphasizing the full reversibility of the switching process.

2 16 0 14 3- [Fe(CN) 6] in supernatant after oxidation -2 12 -4 10 -6 i [µA] i [µA] 8 -8 6 -10 4 -12 2 4- [Fe(CN) 6] in supernatant bevore switching -14 0 4- [Fe(CN) 6] in supernatant after reduction -16 -2 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 E vs. Ag/AgCl [V] E vs. Ag/AgCl [V]

Figure S8: Hydrodynamic voltammograms of the supernatant of the initial solution of 1 mM K 4[Fe(CN) 6] in 0.025 M KCl in presence of 6.3 g/L P(NIPAM-co -MAPTAC) microgel before switching and after reduction ((icr = 1, light blue and blue transferred Q = 670 mC,, left) and after oxidation ( icr = 0.75, green, transfered Q = -790 mC, right) at T = 23 °C, scan rate v = 5 mV/s, rotation rate: 100 rpm at a Pt RDE, centrifugation at 30°C.

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The calculated initial charge ratios and experimentally determined nominal charge ratios are given in Table S1. The ncr is highlighting that the charge compensation of the microgel network by the redox-responsive multivalent counterions is nearly the same under the given conditions (excess of KCl, 50 x more Cl -), but as the icr was varied 3- during the switching process, the fraction of the effectively into the microgel incorporated trivalent [Fe(CN) 6] 4- (56%) is higher compared to the entrapped tetravalent [Fe(CN) 6] fraction (40%). These centrifugation experiments and ncr results confirm the mechanistic hypothesis, namely the decrease in size is due to an increasing amount of 3- the [Fe(CN) 6] entrapped inside the microgel.

Table S1: Adjusted initial charge ratio ( icr ) and experimentally determined nominal charge ratio ( ncr ) before switching after oxidation and after reduction at 30°C.

3- 4- icr ncr [Fe(CN) 6] ncr [Fe(CN) 6] Before switching 1 0.40 After oxidation 0.75 0.42 After reduction 1 0.39

References

1. Mergel, O.; Gelissen, A. P. H.; Wünnemann, P.; Böker, A.; Simon, U.; Plamper, F. A., Selective Packaging of Ferricyanide within Thermoresponsive Microgels. The Journal of Physical Chemistry C 2014, 118 (45), 26199-26211.

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